Method for efficiently extracting and separating high-purity tetrodotoxin from pufferfish viscera

ABSTRACT

A method for extracting and separating high-purity tetrodotoxin (TTX) from pufferfish viscera, comprising the following steps: adding an acid alcohol homogenate to pufferfish viscera, adding a certain amount of a flow aid and stirring uniformly, loading into a chromatographic column, eluting with the same acidic alcohol, collecting an eluent containing TTX, and concentrating under reduced pressure to obtain a TTX concentrated solution; further separating using ion exchange resin column chromatography, collecting an eluent containing TTX, and using a nanofiltration membrane to perform concentration or reduced pressure evaporation to dryness to obtain a TTX crude product; and precipitating and crystallising the TTX crude product by means of a solvent to obtain TTX with a purity greater than 98%.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims the priority to the Chinese patent application with the filing No. CN 202110487094.X filed with the Chinese Patent Office on May 4, 2021, and entitled “Method for Efficiently Extracting, Separating and Preparing High-purity Tetrodotoxin from Pufferfish Viscera”, the contents of which are incorporated herein by reference in entirety.

TECHNICAL FIELD

The present disclosure relates to the technical field of extraction, separation and purification of biologically active substances in animal bodies, and in particular, to a preparation method of tetrodotoxin from pufferfish viscera.

BACKGROUND

Tetrodotoxin (TTX) is originally a small-molecule and non-protein alkaloid with strong neurotoxin extracted from pufferfish. In 1909, Japanese scholar Tawara extracted crude toxin from pufferfish ovaries for the first time and named the same (Bane et al., 2014). In 1950, Yokoo obtained purified TTX crystals from Takifugu rubripesovaries for the first time (Yokoo, 1950). In 1964, the molecular formula (C₁₁H₁₇N₃O₈) and the chemical structure of TTX were determined (Buchwald et al., 1964; Woodward and Gougoutas, 1964; Tsuda and Ikuma et al., 1964). Over a long period of time, tetrodotoxin was considered as a unique toxic substance in the body of pufferfish, and this had not been changed until American scientists Mosher et al. separated TTX from Tarichatorosa in 1964 (Mosher et al., 1964). Studies to date had shown that TTX not only exists in bodies of many kinds of pufferfishes, but also widely exists in nearly 140 kinds of animals, including vertebrates and invertebrates (Lorentz et al., 1016), such as gobies (Zhu Guoping, et al., 2015), newts (Yotsu-Yamashita et al., 2012), bivalves and gastropods (Laura et al., 2019), blue-ring octopus (Whitelaw et al., 2019), nemerteans (Vlasenko et al., 2018), nematodes (Mao Jie, et al., 2020), etc. Moreover, it had been found that many kinds of microorganism can produce TTX (Chau et al., 2011; Magarlamov et al., 2017).

TTX is one of the most toxic neurotoxins known, and its toxicity to humans is around 1200 times that of cyanide, and there is no known antidote (Lago et al., 2015). The LD₅₀ value of TTX to mice is 10 µg/kg (intraperitoneal), 16 µg/kg (subcutaneous), and 332 µg/kg (oral) (Kao, 1966). After ingestion of TTX by humans, the severity of poisoning symptoms is related to the dosage (Homaira et al., 2010). Initial symptoms include lingual and lip numbness (paresthesia), followed by headaches or vomiting, and muscular weakness and ataxia may be developed. In severe cases, death may be caused by respiratory failure or heart failure (Noguchi and Eebesu, 2001). The lethal dose of TTX to humans is 1.33 µg/kg (Kasteel and Westerink, 2017). The problems such as that TTX is transmitted by means of biological chain and a large number of marine and freshwater farmed animals contain TTX cause people to worry about and pay attention to the safety of environment.

TTX is a blocking agent offast-voltage gated sodium channel, can cause nerve and muscular anesthesia, and can be medically used for local anesthesia and addiction-free analgesia of various patients with cancer and having under gone surgery, etc. As a local anesthetic (Ogura and Mori, 1968), TTX does not have obvious cardiovascular side effects (Butterworth, 2010; Stoetzer et al., 2016) or myotoxicity (Padera et al., 2006). Unlike local anesthetics commonly used clinically, TTX has a binding site located outside the membrane, and it can be used in combination with other local anesthetics to produce a synergistic effect (Kohane et al., 1998; Berde et al.,2011; McAlvin et al.,2015). Researchers also used the anesthetic property of TTX to treat various types of pains, such as severe cancer pains (Hagen et al., 2008;2017), or relieve opioid withdrawal syndromes (Chen Suqing, et al., 2001; Kohane et al., 2003; Shi et al., 2009) and so on. In addition, researchers found that tumor growth is obviously reduced in carcinoma-bearing mice treated with TTX (El-Dayem et al., 2013). TTX can also inhibit the mobility or invasiveness of tumor cells, and prevent the metastasis of highly metastatic tumors and leukemias (Shan et al., 2014; Stock and Schwab, 2015). TTX is expected to be clinically applied in many fields.

TTX has a complex structure and is difficult to synthesize chemically. Although total artificial synthesis of TTX has been accomplished as early as in 2003 (Ohyabu et al., 2003), there are too many steps of artificial synthesis, 23-67 steps are required on average, and the obtained yield content is only 0.34%-1.82% (Chau and Cuyfikub, 2011; Makarova et al., 2019). Moreover, in the synthesis process, it is necessary to develop a series of purification methods. Therefore, it is difficult to achieve large-scale production by laboratory synthesis of TTX and analogs thereof (Chau and Cuyfikub, 2011; Bane et al.,2014; Makarova et al.,2019). Although many microorganisms can produce TTX, the yield is quite low (Lago et al., 2015), the mechanism of production thereof is unclear, and to improve its yield and to develop a purification method are both difficult problems to solve (Liu Yanting, et al., 2008). Thus, TTX extracted and prepared from animal tissues is currently the major source of TTX.

TTX is an aminoperhydroquinazoline compound (pKa 8.76), has a molecular formula of C₁₁H₁₇N₃O₈, a molecular weight of 319.27, and strong absorption at 190-200 nm, and contains a cage-shaped orthoester structure (Moczydlowski, 2013), as shown in FIG. 1 . Almost all carbon atoms in the molecules thereof have an asymmetric substitution, and 1,2,3-guanidylamino and hydroxyl groups on C-4, C-9, and C-10 in the vicinity thereof are active groups. The TTX crystal is white in color and odorless, does not have a fixed melting point, and is gradually carbonized at 220° C. or above (Cui Jianzhou, et al., 2005), but does not melt even at 300° C. (Tsuda and Kawamura, 1952). Pure TTX is insoluble in water and most organic solvents, such as methanol, ethanol, and dimethyl sulfoxide, is easily dissolved in dilute acetic acid solution, and the activity thereof is easily destroyed in strong acid or strong alkaline solutions (Moczydlowski, 2013). TTX is positively charged in a neutral solution, and under weakly acidic condition, orthoester form, lactone form, 4-epi TTX, and 4,9-anhydro TTX coexist (Nishikawa and Isobe, 2013).

Formula 1 Structural Formula of Tetrodotoxin (TTX)

TTX has a unique structure and properties, and high-purity TTX can hardly be extracted, separated, purified, and prepared from biomaterials by conventional separation and purification methods for natural alkaloids (Makarova et al., 2019). Initially, Tawara (1909) separated TTX from aqueous extracts of pufferfish ovaries by a lead acetate precipitation method, but the purity was only 0.2%-4% (Moczydlowski, 2013). Despite many attempts, it was not until the early 1950s that Yokoo realized the preparation of pure crystalline TTX through alumina column chromatography and recrystalization after continuously treating crude tetrodotoxin with phosphotungstic acid, mercury picric acid, bitter ketonic acid, phenylhydrazine, etc. (Yokoo, 1950). Subsequently, Tsuda and Kawamura separated and purified TTX by taking 1000 kg of pufferfish ovaries as raw materials, through potato starch partitioning chromatography, activated carbon adsorption chromatography, and filter paper chromatography (Tsuda and Kawamura, 1952).

At present, a process of preparing TTX generally includes extracting pufferfish visceral tissue staken as raw materials, with an aqueous acetic acid solution as an extraction solvent, assisted by ultrasonic waves or/and heating. The extract is purified by cation exchange column chromatography-activated carbon column. TTX is precipitated through acid-dissolution and alkali-precipitation, and finally, the precipitated TTX is purified and recrystallized by preparative high performance liquid chromatography (HPLC). Main reports concerning the extraction and purification method of TTX are summarized in Table 2.

TABLE 2 Extraction and Purification Process for TTX Reported in Literatures Raw material Extraction method Separation and purification method Purity References Pufferfish ovaries or/and livers acid-water immersionextracti on microfiltration→ultrafiltration→nano-filtration→ reverse osmosis→HPLC preparation > 99% Yi Ruizao, et al., 2002 Pufferfish ovaries immersionextracti onby water or weak organic acid aqueous acetic Ambedite IRC-60 cation exchange column chromatography→activated carbon adsorption→crystallisation D201 macroporous resin→Ambedite > 80% Zhou and Shum, Pufferfish livers acid solution with pH of 4.0, boiled to extract IRC-86 cation exchange resin column→ultrafiltration→sephadex G-10 purification→HPLC preparation →crystallisation 99.5 % 2003 Cui Jianzhou,et al., 2005 Pufferfish viscera 50% aqueous ethanol solution (containing 0.1% acetic acid), boiled to extract activated carbon column→D152 cation exchange column→Bio-GelP2 polyacrylamide molecular sieve chromatography→crystallisation > 80% Su Jie, et al., 2010 Gastrophy-sus lunaris ovaries 0.1% aqueous acetic acid solution, ultrasonic-assisted extraction D152 ion exchange resin→activated carbon →Bio-Gel P2 gel column chromatography 46.6 % Huang Zhimei, 2010 pufferfish roes or livers ultrasonic water extraction ion exchange→ ultrafiltration→ nanofiltration →HPLC preparation→ crystallisation > 99% Wang Xiuju, et al., 2012 Pufferfish viscera immersionextracti onby aqueous acetic acid solution dialysis→reverse weak anion chromatography separation→cation exchange chromatography purification→freeze-drying > 80% Yang Yuhui, 2015 Pufferfish livers and ovaries pure water immersionextracti on polymerized ferric sulfate flocculation→D152 cation exchange column chromatography→low pressure silica gel column chromatography →HPLC preparation→crystallisation > 99% Zhong Xinjia, Liu Yandong, 2016 Pufferfish viscera aqueous acetic acid solution, heated to extract activated carbon decolorization→ macroporous resin adsorption→cation exchange resin column chromatogra-phy → nanofiltration→ crystallisation 99.5 % Zeng Junjie, et al., 2018 Nassariida-e 1% acetic acid methanol solution, 60° C. waterbath and ultrasonic-assisted extraction WCX solid phase extraction→D152 cationic resin→Sephadex G-10 purification →HPLC preparation 32.6 % Sun Bolun, 2018 Gastrophy sus lunaris ova ries and livers 0.1% aqueous acetic acid solution, 40° C. waterbath and ultrasonic extraction WCX weak cation exchange column → polyamide chromatographic column → freeze-drying → recrystallisation > 90% Deng Xingcha o, et al., 2018 Nassariida-e acetic acid, methanol and ultrasonic water bath extraction carbon-nanotube solid phase extraction →HPLC preparation > 99% Zhao Donghao, et al., 2019 Pufferfish viscera Stirring and soaking in acetic-acid-containing alcohol-water mixed solution activated carbon decolorization→ macroporous adsorption resin purification→microfiltration → ultrafiltration→nanofiltration→ recrystallisation 97-99% Zhang Xiaojun, et al., 2020

However, these extraction and purification methods have problems in the following several aspects.

-   1) Low extraction efficiency. TTX can hardly be extracted from     tissues and extracted completely, and the conventional soaking     extraction or ultrasonic-assisted extraction methods have low     extraction efficiency and require a large amount of extraction     solvent. -   2) Acidic water is mostly used as the extraction solvent, which     results in a large amount of water-soluble substances such as     proteins contained in the extract solution, so that the proteins     need to be removed in the subsequent TTX separation and purification     processes, and it is difficult to carry out separation and     purification. Meanwhile, there are many separation and purification     steps, such that the recovery rate is very low. -   3) Most reports are limited to laboratory researches, and     experimental methods and results are not verified, and can hardly be     replicated and used for the production of TTX.

Therefore, it is quite important to develop a simple and efficient TTX extraction, separation, and purification method and process that can be used for mass production.

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SUMMARY

The present disclosure aims at providing a method for extracting, separating and preparing high-purity tetrodotoxin from pufferfish viscera with high extraction efficiency, low equipment investment, and convenient operations.

In order to achieve the above technical objectives, a technical solution used in the present disclosure is as follows:

a method for extracting, separating and preparing high-purity tetrodotoxin from pufferfish viscera, which includes the following steps:

-   (1) cutting fresh or thawed pufferfish visceral tissues into small     pieces, adding a certain volume of acidic alcohol, and homogenizing     the resultant in a homogenizer; -   (2) adding a certain amount of a flow aid or a filter aid into a     homogenate obtained in the above (1), stirring and mixing the     resultant uniformly, and then loading the extract into a     chromatographic column according to a column packing method of     column chromatography; after standing for a certain period of time,     eluting TTX in the materials successively by a column chromatography     eluting method using the same acidic alcohol as above, measuring a     TTX contents in the eluted fractions by HPLC until there is no TTX     in the eluent, and collecting the TTX-containing eluent as a TTX     extract solution; -   (3) concentrating the TTX extract solution in the above (2) to a     water phase under reduced pressure in a vacuum concentrator,     recovering ethanol for reuse; and performing extraction on the water     phase with an organic solvent to remove fat-soluble substances,     where in the water phase obtained is a TTX extract concentrated     solution; -   (4) separating the TTX extract concentrated solution obtained in the     above (3) by pre-treated ion exchange resin column chromatography,     after sample loading, first eluting impurities with neutral ammonium     salt and water successively, then eluting TTX with an acidic     alcohol, collecting TTX-containing eluent, concentrating the     TTX-containing eluent to a small volume by a nanofiltration     concentrator, and then concentrating the same to dryness by a vacuum     concentrator, to obtain a TTX crude product; and -   (5) subjecting the TTX crude product obtained in the above (4) to     repeated acid-dissolution and alkali-precipitation or     crystallisation multiple times with water and organic solvent, to     obtain high-purity TTX. The purity of the TTX is 98% or higher upon     HPLC analysis.

In some embodiments, in step (1), the pufferfish viscera are pufferfish ovaries or livers, after the ovaries or livers are cut into small pieces, 95% acidic alcohol with a volume 2.0 times the volume of the viscerais added, and the pufferfish visceral tissues are homogenized in a homogenizer.

In some embodiments, in step (2), the flow aid or the filter aid added in the homogenate is diatomite. The diatomite with a weight 0.5-1.0 times the weight of the viscera is added to the homogenate of the pufferfish visceral tissues, after they are uniformly stirred and mixed, the mixture is made to slowly flow into the chromatographic column. After standing for 1 hour, and all of the TTX in the materials is dissolved, the TTX therein is extracted by elution with 95% acidic alcohol until there is no TTX in the eluent.

In some embodiments, in step (3), the TTX extract solution is concentrated to a water phase under reduced pressure in a vacuum concentrator, and ethanol is recovered for repeated use. The water phase is extracted with an organic solvent to remove fat-soluble substances, and the water phase obtained is a TTX aqueous solution.

In some embodiments, in step (4), the TTX aqueous solution is separated by pre-treated or regenerated D-152 weakly acidic cation exchange resin column chromatography. After sample loading, impurities are eluted with neutral ammonium salt, pure water, a low-concentration acetic acid (0.1-1.0%) solution in sequence, and then the TTX is eluted with a 3-5% acetic acid solution.

In some embodiments, in step (5), the TTX eluent is concentrated to a small volume by a nanofiltration membrane concentrator with molecular weight cut-off of 100-300, and the concentrated solution is further evaporated to dryness by a vacuum concentrator. The concentrate obtained is eluted with alkaline alcohol and alkaline water to remove impurities, and then dissolved in acid water. The solution is adjusted to alkaline (pH 8-9), and crystallized in a 4° C. refrigerator to obtain the high-purity TTX.

Compared with the prior art, the present disclosure has following advantages.

1) For the preparation method of extraction and separation of TTX of the present disclosure, the extraction step therein adopts a novel, efficient and energy-saving column chromatography extraction method, and the extraction process is completed in one step, thus the extraction efficiency is higher. All of the TTX in the raw materials can be completely extracted with the extraction solvent having a volume 7 times that of the raw materials.

2) For the preparation method of extraction and separation of TTX of the present disclosure, the acidic high-concentration alcohol is used as the extraction solvent to perform column chromatography extraction, the extract solution has few water-soluble impurities and does not contain macromolecule water-soluble substances such as proteins, thus greatly simplifying the subsequent TTX separation and purification. Meanwhile, the extraction solvent can be recovered and reused.

3) The preparation method of extraction and separation of TTX of the present disclosure has a simple process, few steps, a high recovery rate of separation and purification, and a high overall yield of the TTX pure product. Therefore, the preparation cost of TTX is low.

4) The equipment used in the preparation method of extraction and separation of TTX of the present disclosure is common column chromatography and concentration equipment or the like, thus the equipment investment is very low.

5) For the preparation method of extraction and separation of TTX of the present disclosure, the extraction, separation and purification processes are all carried out at a room temperature; and the amount of the organic solvent is small, and the organic solvent can be recovered and reused, thus being energy-saving and environment-friendly.

BRIEF DESCRIPTION OF DRAWINGS

For ease of explanation, the present disclosure is described in detail by the following examples and accompanying drawings.

FIG. 1 shows a flow diagram of a process of extracting, separating, and preparing TTX of the present disclosure.

FIG. 2 shows HPLC analysis of a prepared TTX sample.

DETAILED DESCRIPTION OF EMBODIMENTS

A process flow of the “method for efficiently extracting, separating and preparing high-purity tetrodotoxin from pufferfish viscera” of the present disclosure is as follows.

15 kilograms of pufferfish ovaries, after being rinsed with fresh water and undergoing impurity removal, were disintegrated by a meat grinder, acid (1% acetic acid), anhydrous ethanol having a weight 2-3 times that of the pufferfish ovaries and diatomite having a weight 0.6 times that of the pufferfish ovaries were added, the mixture was stirred uniformly, and uniformly loaded into a chromatographic column of ø 200 × 1500 mm, after standing for 1 hour, the mixture was eluted with acidic ethanol until there was no TTX in a chromatographic eluent. The chromatographic eluent, i.e., TTX extract solution, was collected. The extract solution containing TTX of 1236.0 mg was obtained, and TTX extraction yield was 96%.

The TTX extract solution was concentrated to a water phase under reduced pressure, and the recovered ethanol can be used repeatedly. The concentrated solution was extracted with acetic ether having a volume ½-⅒ times that of the concentrated solution to remove low-polarity impurities and obtain a TTX extract concentrated solution. The extract concentrated solution containing TTX of 1168 mg was obtained, and a TTX recovery rate was 90.5%.

The extract concentrated solution of 15 kilograms of pufferfish ovaries was separated by D-152 weakly acidic cation exchange resin column chromatography (ø 100 × 1000 mm). After sample loading, the sample was eluted at 3BV with a 0.2% ammonium phosphate solution, pure water, and 0.5% acetic acid water in sequence, such that impurities were washed away. Then, the resultant was eluted with a 2% aqueous acetic acid solution until there was no TTX, and the recovery rate of TTX was 93.6%. The eluent fractions with high TTX content were collected, concentrated to a small volume by nanofiltration, and concentrated to dryness under reduced pressure to obtain a TTX crude product, wherein the recovery rate of the TTX was 90.4%. An eluted phase with a low TTX content was then concentrated by D-152 weakly acidic cation exchange resin column chromatography to recover the TTX.

The crude product was eluted 3 times with alkaline ethanol and alkaline water, and the precipitation was subjected to 3 times of acid-dissolution and alkali-precipitation repeatedly, to obtain purified TTX, wherein the recovery rate of TTX was 80.7%.

The TTX was subjected to HPLC analysis and the content was measured, to obtain 815.6 mg of TTX. Upon HPLC analysis, the TTX purity was ≥ 98% (as in FIG. 2 ). The total recovery rate of TTX was 63.3%. 

What is claimed is:
 1. A method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera, comprising steps of: (1) cutting fresh or thawed pufferfish visceral tissues into small pieces, adding a certain volume of acidic alcohol, and homogenizing a resultant in a homogenizer; (2) adding a certain amount of a flow aid or a filter aid into a homogenate obtained in the (1), stirring and mixing a resultant uniformly, and then loading the resultant into a chromatographic column according to a column packing method of column chromatography; after standing for a certain period of time, completely eluting TTX in materials by a column chromatography eluting method using the same acidic alcohol, measuring a TTX content in the eluent fractions by HPLC until no TTX exists in the eluent, and collecting the TTX-containing eluent fractions as an extract solution; (3) concentrating the TTX extract solution in the (2) to a water phase under reduced pressure in a vacuum concentrator, recovering ethanol for repeated use, and performing extraction on the water phase with an organic solvent to remove fat-soluble substances, wherein a water phase obtained is a TTX extract concentrated solution; (4) separating the TTX extract concentrated solution obtained in the (3) by pre-treated ion exchange resin column chromatography, after sample loading, first eluting impurities with neutral ammonium salt, pure water and low acidic solution successively, then eluting TTX with an higher acidic aqueous solution, collecting an eluent phases having a high TTX content, and concentrating the TTX eluent phases by nanofiltration or under reduced pressure to dryness, to obtain a TTX crude product; and (5) subjecting the TTX crude product to repeated acid-dissolution and alkali-precipitation or crystallisation multiple times in an aqueous solution or organic solvent, to obtain high-purity TTX, wherein a purity of the TTX is 98% or higher upon HPLC analysis.
 2. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, where in the step (1), the pufferfish viscera are tetrodotoxin-containing pufferfish ovaries, livers, skins or other tissues.
 3. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, where in the acidic alcohol in the step (1) is a lower fatty alcohol more than 50%, including but not limited to methanol, ethanol, ethylene glycol, etc.
 4. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, wherein the flow aid or filter aid in the step (2) is a substance for improving fluidity of a liquid in an animal tissue homogenate, including but not limited to the minerals such as diatomite, perlite, and acidic clay, sawdust, cellulose, and activated carbon, etc., and an added amount is 0.5-2.0 times a weight of the pufferfish viscera.
 5. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, where in the step (2), the flow aid is added to the homogenate, and column packing after the uniform stirring and mixing, impurity elution, and TTX elution methods and processes are carried out completely according to a method of column chromatography.
 6. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, wherein in the step (3), the TTX extract solution is concentrated to the water phase under reduced pressure at about 65° C. in the vacuum concentrator, and recovered ethanol is repeatedly used for extraction of the TTX.
 7. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, wherein in the step (3), the water phase is extracted with the organic solvent to remove the fat-soluble substances, the organic solvent used is petroleum ether, acetic ether or other organic solvents that can perform extraction with an aqueous solution, and the water phase obtained after the extraction is the TTX extract concentrated solution.
 8. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, wherein in the step (4), the extract concentrated solution is separated by the pre-treated ion exchange resin column chromatography, and the ion exchange resin is D-152 weakly acidic ion exchange resin, etc.
 9. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, wherein in the step (4), the TTX eluent separated by a cation exchange resin column chromatography is concentrated to a small volume by a nanofiltration membrane, and the nanofiltration concentration membrane is a nanofiltration member with molecular weight cut-off smaller than 300 Dalton (generally, the molecular weight cut-off is 100 \~200 Dalton).
 10. The method for efficiently extracting, separating and preparing tetrodotoxin from pufferfish viscera according to claim 1, wherein in the step (5), an acid in the acid-dissolution and alkali-precipitation or crystallisation of the TTX crude product is a weak acid such as acetic acid or phosphoric acid of about 1%; and a base is a weak alkaline solution such as ammonia water, with a pH value of 7~10. 